The problem of long-distance data transmission from or to a mobile terminal arises, in particular, in the case of links between computers (known as machine to machine links). This field of transmission is characterized by a data rate requirement which is markedly lower than that of image or Internet links, or alternatively by a capacity to use an intermittent link rather than a continuous one.
In this case, the satellite serves as a communications link between the two parties, and the latency time of acknowledgements and messages is a function of the distance between the satellite and the ground station (also known as the gateway Earth station, or GES).
However, the coverage provided by the network of ground stations of systems using low-orbit satellites such as Orbcomm (registered trademark) and Argos (registered trademark) is limited by the deployment of ground stations (GES), and the existing systems provide only a limited coverage of the Earth in this mode. In fact, each ground station allows coverage over a radius of about 3000 km, and each of these systems includes about twenty ground stations.
Clearly, therefore, the coverage areas include large “blank” areas in which the system cannot be used. These areas include, in particular, a large proportion of the oceanic regions, and even a significant part of continental regions such as Africa or Australia.
In cases where the LEO satellite has no simultaneous view of the user terminal and the ground control stations (GES), it is necessary to use a communications method of the type known to those skilled in the art as “store & forward”. In this method, the message is stored on the satellite, which continues to travel in its orbit until it overflies the ground station GES to which it delivers the stored message.
With this method of operation, the call delays are long and create difficulties in making bidirectional calls in acceptable conditions, since the delays are typically between a few minutes and the 100 to 150 minutes required for a complete orbit of the LEO satellite.
There are also known examples of hybrid telecommunications systems for data transmission between users. A hybrid system of this type is composed of geostationary satellites and a constellation of low-orbit satellites.
Notably, a first patent document, FR 2764755/U.S. Pat. No. 6,208,625, may be cited: Method and apparatus for increasing call-handling capacity using a multi-tier satellite network
This document describes a network formed by LEO and geostationary (GEO) satellites capable of communicating with each other. On the ground, user terminals can carry out reception/transmission (Rx/Tx) operations with the LEO and GEO satellites. The LEO component provides filtering of the traffic received from the terminals, and switches this traffic, as a function of the urgency of the traffic received, either internally toward the LEO system or toward the GEO system.
In a second patent document, EP 0883252/U.S. Pat. No. 6,339,707, entitled Method and system for providing wideband communications to mobile users in a satellite-based network, a satellite communications system is proposed for providing global coverage, reduced delay in transmission (Tx), and maximization of system capacity utilization (wideband satellite communications by the interconnection of a plurality of medium-orbit (MEO) and geostationary (GEO) constellations).
The MEO and GEO satellites communicate with each other directly by inter-satellite links, permitting traffic routing (for voice and data) on board the satellites according to certain rules.
This document also proposes spectrum sharing and re-use between the GEO and MEO satellites at very high frequencies (between 40 and 60 GHz, for example), to allow what is known as the “seamless handover” function for portable terminals (that is to say, switching from a mobile to a fixed network without interruption of the current call).
Clearly, the present hybrid systems have a high degree of complexity, with correspondingly high costs of installation and use.
The problem posed by this type of service is that it is desirable to offer a high-quality service at a very low price for users, implying a reduction in the cost of the system without degradation of its performance.
However, the price of the space segment in this type of system may be significant, especially if the satellites used are complex and/or numerous. It is therefore necessary to maximize the directivity and the link budget offered to users over the whole terrestrial surface while using satellites having the smallest possible degree of complexity.